I can't pick just one best but I would include on the list:
Showboat
Deliverance
To Kill a Mockingbird
Night of the Hunter (original with Robert Mitchum)
Winter's Bone
made for television:
Lonesome Dove
Band of Brothers
Marty
The Duel
Well it's been going on for a long time but anything by Edgar Rice Burroughs has been a huge disappointment. If you don't recognize the name, Burroughs was the author of the Tarzan books, Martian and Pellucidar series, and various unrelated novels. Of course, they are pulp fiction, but his red-blooded heroes and heroines become sappy when interpreted by Hollywood.
I loved the Ring Trilogy because the book IMHO had too much trudging on and on and on and on-- the screen version moved much better. On the other hand, two of my favorite sections of the novel were Tom Bombadill's (sp) and the chapter "Cleansing of the Shire." I hope the Hobbit comes up to the standard set by the Ring Trilogy.
Oh, and True Grit. Mattie was a great female character but Hollywood had to make the males more prominent. Both times. :thumbdown: